Grow dense, make sense?

A Baltic Bridge project to investigate the role of drift algae, aquatic plant density and traits for secondary production and shelter from predation.

Åsa and Charlotte are measuring the distance from their sampling spots to bladderwrack. They observed schools of sticklebacks by the bladderwrack and thought that the distance to bladderwrack might affect the survival of the amphipods.

In this Baltic Bridge experiment Åsa and Charlotte have started to investigate if the density of aquatic vegetation and drift algae, and/or the vegetation traits, affects the composition of the macroinvertebrate community and if the macroinvertebrates’ risk to be eaten by fish.

Shallow inlets are important nursery grounds for fish fry. Here, the fish fry can hide in dense vegetation, and there are often many small animals to eat. Earlier studies have shown that the risk for small animals being eaten by fish decrease if they hide in dense vegetation. But there are also studies showing that the density of vegetation does not affect the risk for small animals to be eaten.

Field work at Askö in June & August

In this experiment, we wanted to investigate if other factors, such as the amount of filamentous algae and plant traits, can affect the community composition of small animals that live among the vegetation, and their risk of being eaten by fish. We have used a common method to measure predation (to be eaten), where you set out amphipods in different densities of vegetation, and look at how many survive the night. Since the predation of amphipods was extremely high in June, when we also caught a lot of sticklebacks, we had to count how many amphipods had been eaten already after one hour, before they were all eaten.

While estimating the cover of different vegetation species using the red frame in the picture, a stickleback was found to be eating one of the amphipods attached to a fishing line that was glued to the plastic rod.

In addition, we took a closer look at the community of fish, filamentous algae, vegetation and small animals to see what best explains the amount of eaten amphipods. The predation of amphipods was extremely high in June, when we also caught a lot of sticklebacks. Therefore we had to count how many amphipods had been eaten already after one hour, before they were all eaten.

Fish traps used for estimating the abundance of small fish at the different sampling spots, which were placed in different densities and types of vegetation. The traps were deployed in the evening and retrieved the following morning.